This invention relates generally as indicated to a rotatable fill pipe having a collapsible backfill elbow at the extreme lower end thereof, and to the method of using such fill pipe to blind-fill underground voids from the surface to prevent surface subsidence.
Heretofore, it was the usual practice to blind-fill large underground cavities, stopes, voids, mined-out areas, etc. (hereafter underground voids) by discharging hydraulic and pneumatically placed fill vertically from the surface to the bottom of the void. This had the disadvantage that the fill material formed a cone shaped pile at the bottom of the void which limited the quantity of fill that could be introduced into the void.